Means for preventing the adhesion of mineral fibers to the walls thereof of a forming chamber

ABSTRACT

A forming chamber or hood overlying an air-permeable support with a source of vacuum therebelow, which creates a downwardly directed stream of gas in the chamber into which is dropped electrostatically charged mineral fibers having a tendency to adhere to the walls of the chamber as they are distributed on the support for the formation of a felt or mat of the deposited fibers. The adhesion of the fibers to the chamber walls is prevented by the provision of a plurality of openings in the chamber walls of predetermined diameter, disposition and spacing relative to a slight underpressure maintained in the interior of the chamber, through which openings are sucked in a plurality of slight air currents from the outside to the inside of the chamber walls, whereat they are deflected in a downward direction by the gas stream to form air cushions along the walls on which the fibers slide downwardly without touching the walls of the chamber and without the generation of vortices therein.

United States Patent Jaeger 1 Jan. 25, 1972 54] MEANS FUR PREVENTING THE 3,170,197 2/1965 Brenner 19/156 x 3,177,275 4/1965 Brenner .1

ADHESION OF MINERAL FIBERS To 3,226,773 l/1966 Paliyenko ..19/66 THE WALLS THEREOF OF A FORMING CHAMBER [72] Inventor: Erwin Jaeger, zum Kalverhof, Germany [73] Compagnie de Saint-Gobain, Neuilly-sur- Seine, France Apr. 14, 1969 Assignee:

Filed:

Appl. No.:

[30] Foreign Application Priority Data Apr. 16, 1968 Germany ..P 17 60 261.8

[56] References Cited UNITED STATES PATENTS 5/1943 Drill ..l9/l56 UX ll/l952 Sheidley. ...l9/l56.3 X 4/1963 Marley ..l9/.46

Primary Examiner-Dorsey Newton Attorney-Dale A. Bauer, John L. Seymour, Bauer and Seymour and Samuel Lebowitz [57] ABSTRACT A forming chamber or hood overlying an air-permeable support with a source of vacuum therebelow, which creates a downwardly directed stream of gas in the chamber into which is dropped electrostatically charged mineral fibers having a tendency to adhere to the walls of the chamber as they are distributed on the support for the formation ofa felt or mat of the deposited fibers. The adhesion of the fibers to the chamber walls is prevented by the provision ofa plurality of openings in the chamber walls of predetermined diameter, disposition and spacing relative to a slight underpressure maintained in the interior of the chamber, through which openings are sucked in a plurality of slight air currents from the outside to the inside of the chamber walls, whereat they are deflected in a downward direction by the gas stream to form air cushions along the walls on which the fibers slide downwardly without touching the walls of the chamber and without the generation of vortices therein.

' 15 Claims, 5 Drawing Figures Pmmiumzsmz 3336590 SHEET 1 0F 3 me 11? if Ame INV ENT OR few/A1 JAE'GER ATTORNEY PATENTEDJANZSISYE SHEET 3 BF 3 u G F.

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ERWIN JAEGER MEANS FOR PREVENTHNG THE ADHESHON OF MINERAL FIBERS TO THE WALLS THEREOF OF A FORMING CHAMBER flow of gas, and into which the fibers or strands are introduced 1 from above, and in which they are deposited on a support at the bottom, as a fleece, felt, mat or the like, below which is provided a suction device. The invention is directed particularly to the prevention of the adhesion resulting from an electrostatic charge accumulated by the filaments, strands, or staple fibers. The invention is also concerned with the development of a suitable forming chamber or hood for the execution of the above method.

In the production of, for example, glass fiber mats, the glass threads pulled off by the spinning or roving bobbins are cut into staple strands of desired length, by a cutting device mounted above the forming chamber. The cutting device usually consists of two rollers revolving in opposite directions, one of which is equipped with cutting knives. The staple fibers are introduced into the chamber through an input funnel provided at the top thereof below the cutting device, and are spread by a distributor revolving below the funnel, across the width of the support. It has been found that the threads or fibers are charged electrostatically more or less strongly by virtue of the deflections experienced by the threads during the cutting and their striking against the distributor, and particularly in the case of certain types of the sizing which may be applied to the threads. As a result of this charge on the threads, a part of them, after their entry into the chamber, adhere to the inside walls of the latter. Additional fibers adhere to these fibers, until the accumulation of fibers becomes so heavy that they slide down along the walls of the chamber and form clearly discernible thicker places on the mat and consequently greatly lower the quality of the latter.

A method and apparatus are known, as disclosed in US. Pat. No. 2,697,056, Dec. 14, 1954, according to which glass fibers produced by combustion gases and air currents from glass rovings are blown into the forming hood through an opening provided in the sidewall thereof and are deposited on a conveyor belt therein equipped with a suction device therebelow. A liquid binder of an organic type is sprayed, by means of nozzles, onto the fibers within the hood. In order to prevent the accumulation of an electrostatic charge on the fibers, additional nozzles are provided in the hood, through which water is sprayed onto the fibers. Thereby the electrostatic charge of the fibers is lowered, which prevents the adhesion of the fibers to the walls of the hood and an agglomeration of the fibers.

This known process is disadvantageous in that the fibers now will adhere to the walls of the hood because of the high degree of moisture therein rather than because of the electrostatic charge. The adhesion of the fibers to the walls of the hood will thus not be prevented, but it will even possibly be increased further. Therefore, in the case of this process as well, the above described disadvantage exists. Additional fibers will adhere to the fibers which have become attached to the walls of the hood, until the agglomerations of fibers have become so heavy that they will slide down along the walls of the hood and will form clearly discernible thicker places on the mat, and thereby lower the quality of said mat considerably.

Furthermore, it is known, as disclosed in U.S. Pat. No. 3,245,938, Apr. 12, 1966, that during the treating of the glass fibers which have been provided with organic coatings, antistatic substances may be added to these coatings in order to lower the electrostatic charge. However, it has been found through experiments that this treatment is not sufi'icient to prevent an adhesion of the fibers to the walls of the chamber. Besides that, the addition of the antistatic substances has other disadvantages; for example, it has a deleterious effect on speed of impregnation during the impregnation of the produced mat with a synthetic, and on the adhesive strength and the transparency of the laminates of the plastic molded parts which are reenforced with such mats.

It is the object of the present invention to provide a forming chamber and a method of operating it which avoid the objections encountered previously and especially those described above. Specifically, it is the aim of the instant invention to prevent the adhesion of fibers, staple yarns and the like to the walls of the forming chamber or hood in a simple, reliable and economical manner, and thus to produce a fleece, felt or mat, or like article, of good quality. Beyond that, the invention seeks to improve upon the treatments and the devices known heretofore to attain particularly advantageous results.

The invention is based upon the principle of employing small streams of air which are sucked into the forming chamber through holes in the walls of the chamber by means of an underpressure prevailing in said chamber and which are deflected downwardly by the flow of gas in the chamber in a downward direction, to form air cushions along the walls of the chamber, on which the fibers, strands or staple yarns will slide downwardly without touching the chamber walls.

Experiments have established that the process according to the invention completely avoids the disadvantages described above and that the operation thereof is very reliable. Furthermore, the process according to the invention is exceedingly simple. It requires no additional machine expenditures, for example, such as nozzles for spraying water. It also makes possible operation without the addition of expensive antistatic agents, which, as described above, impair the quality of the final products. Thus, the process according to the invention, has proven more economical than any known heretofore.

A suitable mode of practicing the inventive process is realized by so selecting the cross section of the air currents and the density and position of their places of entry that, in the case of a given underpressure, a uniform air cushion is formed along the walls of the chamber without any vortex developing therein. The underpressure customarily present in the chamber, which is produced through the suction device below the support, is selected with consideration given to the desired deposition of the fibers in as uniform a layer as possible. Through experiments it was found that this underpressure, possibly even without a change in the pressure level, is suitable to allow the uniform air cushion to develop along the walls of the chamber, if the cross section of the air currents and the density and position of their places of entry are selected in such a manner, that altogether these air currents are not so strong as to develop undesirable vortices, which would cause the fibers or staple yarns to be blown away too far from the walls of the chamber, or that the underpressure in the chamber will drop too much, which would disturb the distribution of the fibers. On the other hand, these air currents should not be so weak to the extend that their uneven and weak distribution would prove insufficient for the formation of a uniform air cushion. Favorable conditions can always be determined through experimentation. The following procedure, according to the invention, was derived from such experiments. It has been found practicable to arrange that the diameter of the air currents upon their entry into the chamber, be at least about 2 mm., and in the case of an underpressure in the plenum chamber of the order of magnitude of about 25 mm. to 35 mm. WS. (Water column), and at a maximum of about 6 mm., and preferably about 4 mm., with the displacement between the edges of the air currents at least about 7 mm., and at most, approximately equal to the length of the fibers, strands, or staple yarns. The experiments have established that satisfactory results can be achieved with the above values.

The invention also contemplates a forming chamber or hood adapted to execute the method described above. Such a chamber is equipped in a known manner with an inlet opening at the top for the introduction of the fibers, threads or staple yarns, possibly with a distributor therebeyond, and with a support at the bottom which may be formed of an air-permeable endless belt, perforated screen or the like, which is fitted with a suction device below the support, The chamber according to the invention is characterized by the fact that the walls thereof are provided with holes therein for the entry of air, which are distributed uniformly over a substantial part of their height.

In order to maintain the underpressure in the forming chamber as steady as possible, it is desirable to provide the holes only where the fibers may come into contact with the walls in the course of their dropping movement. Therefore, the upper part of the walls of the chamber which lie above this area, may be formed without any holes. On the other hand, the walls of the chamber may be provided with holes over their entire height, in order to prevent with certainty the adhesion of the fibers or strands to any part of the chamber.

A suitable design of the forming chamber according to the invention is characterized by such a ratio of the diameter of the holes to the distance between them, that at a predetermined underpressure in the chamber, a uniform air cushion is formed along the walls of the chamber without any vortex arising therein. Suitable values for these factors may be determined by experimentation.

The chamber developed on the basis of these experiments is characterized by holes having a diameter of at least 2 mm., and, in the case of an underpressure in the chamber in the order of magnitude of about 25 mm. to 35 mm., (water column), of a maximum of about 6 mm., and preferably about 4 mm., with a displacement between the edges of the holes, which in a horizontal direction is preferably about as large as it is in a vertical direction, of at least about 7 mm., and at the most approximately equal to the length of the fibers, strands, or staple yarns.

If the diameter of the holes is smaller than 2 mm., the quantity of air required for the formation of the air cushion can no longer be sucked in. Besides that, the holes would become clogged very quickly with fiber-binder-dust mixtures, and would no longer be able to fulfill their function. If the diameter of the holes is larger than 6 mm., at an underpressure of about 25 mm. to 35 mm. (water column), then too much air will flow into the chamber. Undesirable vortices of air develop, the fibers or staple yarns are blown away too far from the wall of the chamber, and the underpressure in the plenum chamber becomes too low, or upon an increase in the suction action, the stream of gas in the chamber which is directed downwardly, becomes too strong, at least in parts. The distribution of the fibers is disturbed in all these cases. In respect to the mutual distances between the edges of the holes, it has been found that equal distances in a horizontal and a vertical direction favor the formation of a uniform air cushion along the walls of the chamber. Otherwise, it is suitable to select the distances between the edges of the holes within certain limits as large as possible for the purpose of maintaining satisfactory conditions of airflow in the chamber. A displacement of less than about 7 mm. would render impossible the maintenance of the necessary underpressure which is required in the chamber for the formation of mats. However, if distance between the edges of the holes is too large, then the individual air cushions running along the walls of the chamber no longer touch each other, and strips running from top to bottom will develop. If the distance is greater than the length of the fibers which are fed into the chamber or cut therein, then it might happen that a part of the fibers or staple yarns will not be seized by any of the strips of air cushions and that it will adhere to the wall of the chamber because of its static charge. On the other hand, with a displacement between the edges of the holes which at a maximum is equal to the length of the fibers or the staple yarns, none of the fibers or staple yarns can adhere permanently to the walls of the chamber.

An effective further development of the forming chamber is characterized by a displaced or staggered arrangement of the holes in relation to one another. Thereby, the currents of air entering through the holes intersect each other, as a result of which the air cushion is made even and uniform and the adherence of the fibers to the walls will be prevented with particular certainty.

Another further improved development of the forming chamber according to the invention is attained by arranging the holes in rows, preferably running horizontally, with the diameters thereof decreasing from the upper rows in the direction of the lower rows. This arrangement prevents air currents from developing in the neighborhood of the edge of the mats, which could lead to the formation of mats with edges which are too thin.

This type of forming chamber according to the invention may also be modified by arranging each two rows to form a group with holes of equal diameters, with the diameters of the holes from top to bottom decreasing from group to group. This type of chamber according to the invention is simpler from a point of view of construction, since several rows with holes of equal diameters may be fabricated easily. At the same time, the effect is essentially the same as is achieved with the diameters of the holes decreasing from the top to the bottom, from row to row.

A further suitable improvement of the forming chamber or hood according to the invention is attained by arranging that the holes have a conically enlarged part adjacent to the out side of the walls of the chamber. By virtue of the conical enlargement, less resistance is offered to the inflowing air as a result of the more favorable aerodynamic conditions. Also, it is easier to keep such holes clean from the outside. Furthermore, this construction presents a convenient control factor by modifying the design of the angle and depth of the conical part, since the effective length of the path of flow will control exactly the aspirated air at a given underpressure in the chamber, so that the desired air cushion will develop along the walls of the chamber. Thus, if the conic angle is relatively pointed or acute, the resistance to the flow of air in the hole is relatively large. Therefore, the quantity of air entering through the hole is relatively small. Also, such holes are inclined to be plugged up quickly. Even then, a small depth of the conic part is effective. If, on the other hand, the conic angle is relatively large, under certain circumstances obtuse, and if the conic part has a great depth, then the flow resistance during passage of the air through the appurtenant hole is relatively small, and much air will enter into the chamber, and the resulting air currents may possibly continue into the interior of the chamber before they are deflected in a vertical direction. In this manner, the fibers or staple yarns coming into contact with them are, under certain circumstances, blown away far from the wall of the chamber, and the distribution of the fibers is disturbed, with the result that a mat of lesser quality will be produced. Commensurately selected intermediate values for the conic angle and the depth of the conic part produce the desired results. Altogether, it is evident that the adjustment of the conic angle and of the depth of the conic part presents a possibility for the control or adjustment which may be used in addition or alternatively to the selection of the diameter of the holes or of the distances between them.

Other objects and purposes will appear from the detailed description of the invention following hereinafter, taken in conjunction with the accompanying drawings, wherein FIG. 1 is a schematic sectional view of an apparatus for the production of a glass fiber mat with a forming chamber according to the invention;

FIG. 2 is a vertical sectional view, on an enlarged scale, of a part of the wall of the forming chamber provided with holes; and

FIG. 3 is a front elevation, on the same scale as FIG. 2, of a part of the wall of the forming chamber which has been provided with staggered holes;

FIGS. 3a and 3b show different arrangements of the orifices in the walls of the forming chamber which decrease in cross section from the top downwardly.

FIG. 1 shows a device for the production of a glass fiber mat. This device comprises the forming chamber or hood 1 according to the invention, its lateral walls 2 being provided with holes 3. Funnels 8, 8' are provided at the top of the chamber and the bottom thereof may be constituted by a conveying belt 13. Two chambers 14, I4 are provided below the conveyor belt, which are connected with air suction devices, (not shown), in order to create an underpressure in the forming chamber 1. By virtue of the underpressure prevailing in the chamber 1, slight currents of air 16 are sucked or aspirated into the chamber through holes 3 in the walls thereof, which immediately after their entry into the chamber are directed downwardly because of the effect of the suction devices at the bottom of the chamber 1, and which thus form an air cushion along the walls 2 thereof. Cutting devices may be provided above the funnels 8, 8'. These may consist of two cooperating rollers 5, 6, and 5', 6', respectively, with cutting knives provided in rollers 5, 5'. The strands of glass fibers or rovings 4 or 4', which are fed to the cutting devices, are cut into pieces 7, 7' of the desired length, which, generally speaking, does not exceed 52 mm. The staple yarns 7, 7' drop through funnels 8, 8' onto distributor elements 9, 9' which rotate around a vertical axis, by means of drive shafts 10, 10' and members 11, l 1' which are actuated by the motors 12, 12', respectively. The staple yarns 7, 7', distributed by the distributors 9, 9 across the width of the air-permeable or perforated conveyor belt 13, fall across the width of the belt on the bottom of the chamber to form thereon the mat 15. The yarns which are thrown by the distributors 9, 9' against the walls 2 of the chamber will not adhere to the walls thereof, but will slide down on the air cushion formed by the currents of air 16, to the support 13 without touching at all the walls 2 of the chamber to form the mat 15.

In FIGS. 2 and 3 is shown an arrangement of holes in the wall of the forming chamber where the holes have a conically enlarged part adjacent the outside of the chamber wall. In the embodiment illustrated in the drawing, the conic angle amounts to about 90, and the depth of the conical part takes up about half the thickness of the wall. In the case of a subatmospheric pressure of about mm. to 35 mm. (water column) in the chamber, the resulting flow resistance to the passage of the air currents through the holes in such, that these air currents enter the chamber at such a speed, that they will be deflected in a downward direction immediately after their entry by the general gas current in the chamber, where they will form an air cushion on the inside of the chamber along the walls thereof, on which the cut yarns or thread slide downwardly without touching the walls of the chamber.

The arrangement of holes in the wall of the chamber shown in FIG. 3 is of the same number as those shown in FIG. 2. However, these holes have been arranged in staggered relation, so that the air currents which move downwardly are closer together and form a uniform cushion without necessitating that the holes be disposed at a lesser distance from each other for this purpose.

FIGS. 3a and 3b show arrangements of the holes in the wall of the chamber in staggered relation as shown in FIG. 3, but with the holes decreasing in diameter from the upper rows downwardly. FIG. 3a shows an upper group of five rows of holes having a diameter of 6 mm., an intermediategroup of rows of holes having a diameter of 4 mm., and a bottom group of rows of holes having a diameter of 2 mm. In the alternative arrangement shown in FIG. 3b, the diameters of the holes in successive rows decrease in substantially uniform decrements of 0.3 mm., from the top row having a diameter of 6.0 mm., for example, to 2.0 mm. in the bottom row.

The process according to the invention and the forming chamber for the execution thereof have been described in connection with the production of a glass fiber mat. However, the invention also contemplates corresponding processes, chambers or devices for the production of cushions, felts, mats, plates, preformed shapes and the like, from any other discontinuous mineral or organic fibers.

I claim:

I. A forming chamber for producing a mat of fibers on an air-permeable support at the bottom thereof with suction means below said support, comprising a. peripheral wall portions above said support to define said chamber adapted to be maintained at subatmospheric pressure,

b. an inlet for the fibers at the top of said chamber wherefrom they are distributed onto said support in a downwardly moving gas stream, and

c. said wall portions having a plurality of small orifices around the complete periphery thereof, said orifices being of circular cross section and disposed in a multiplicity of levels above said support in excess of ten, which extend through a substantial height thereof, through which a plurality of small air currents are aspirated into said chamber and thereafter deflected in a downward direction by said downwardly moving gas stream to form air cushions closely adjacent to said wall portions on the interior of said chamber, along which the fibers slide downwardly without contacting and consequently adhering to the walls of said chamber, said openings being so closely spaced from each other that the small current aspirated therethrough forms a continuous cushioning layer for the fibers.

2. An apparatus as set forth in claim 1, including a distributing device for said fibers below said inlet for scattering the fibers substantially uniformly across the area of said support.

3. An apparatus as set forth in claim 2, including rotary cutting means above said inlet for cutting threads and filaments of the fibers into relatively short lengths.

4. An apparatus as set forth in claim 1, wherein said support at the bottom of said chamber comprises an endless conveyor belt.

5. An apparatus as set forth in claim 1, wherein said support at the bottom of said chamber comprises a perforated screen.

6. 'An apparatus as set forth in claim 2, wherein the plurality of small orifices are provided in said wall portions only below said distributing device.

7. An apparatus as set forth in claim I, wherein the small orifices in said wall portion are of conical cross section adjacent to the exterior of the wall portions.

8. A forming chamber for producing a mat of fibers on an air-permeable support at the bottom thereof with suction means below said support, comprising a. wall portions above said support to define said chamber adapted to be maintained at subatmospheric pressure,

b. an inlet for the fibers at the top of said chamber wherefrom they are distributed onto said support in a downwardly moving gas stream,

. said wall portions having a plurality of openings along a substantial height thereof, through which a plurality of small air currents are aspirated into said chamber and thereafter deflected in a downward direction by said downwardly moving gas stream to form air cushions adjacent to said wall portions on the interior of said chamber, along which the fibers slide downwardly without contacting and consequently adhering to the walls of said chamber,

. said openings in said wall portion being of circular cross section of diameters ranging between 2 mm. and 6 mm. with an underpressure in the chamber amounting to approximately 25 mm. to 35 mm. of water column, and correlated in their distribution and spacing so that uniform air cushions are formed on the interior of said chamber without the generation of vortices therein.

9. An apparatus as set forth in claim 8, wherein the diameters of said openings are approximately 4 mm.

10. An apparatus as set forth in claim 8, wherein the spacing between the openings in a horizontal direction is approximately the same as in a vertical direction, said spacing being at least approximately 7 mm. between the edges of the adjacent openings and substantially no greater than the lengths of fibers dropping in said chamber.

11. An apparatus as set forth in claim 10, wherein the spacing between the edges of adjacent openings is substantially no greater than 52 mm.

12. An apparatus as set forth in claim 10, wherein said openings are disposed in staggered relation in successive rows.

diameters of the openings decrease in each successive row.

15. An apparatus as set forth in claim 13, wherein the diameters of the openings in groups of rows decrease in each successive group. 

1. A forming chamber for producing a mat of fibers on an airpermeable support at the bottom thereof with suction means below said support, comprising a. peripheral wall portions above said support to define said chamber adapted to be maintained at subatmospheric pressure, b. an inlet for the fibers at the top of said chamber wherefrom they are distributed onto said support in a downwardly moving gas stream, and c. said wall portions having a plurality of small orifices around the complete periphery thereof, said orifices being of circular cross-section and disposed in a multiplicity of levels above said support in excess of ten, which extend through a substantial height thereof, through which a plurality of small air curreNts are aspirated into said chamber and thereafter deflected in a downward direction by said downwardly moving gas stream to form air cushions closely adjacent to said wall portions on the interior of said chamber, along which the fibers slide downwardly without contacting and consequently adhering to the walls of said chamber, said openings being so closely spaced from each other that the small current aspirated therethrough forms a continuous cushioning layer for the fibers.
 2. An apparatus as set forth in claim 1, including a distributing device for said fibers below said inlet for scattering the fibers substantially uniformly across the area of said support.
 3. An apparatus as set forth in claim 2, including rotary cutting means above said inlet for cutting threads and filaments of the fibers into relatively short lengths.
 4. An apparatus as set forth in claim 1, wherein said support at the bottom of said chamber comprises an endless conveyor belt.
 5. An apparatus as set forth in claim 1, wherein said support at the bottom of said chamber comprises a perforated screen.
 6. An apparatus as set forth in claim 2, wherein the plurality of small orifices are provided in said wall portions only below said distributing device.
 7. An apparatus as set forth in claim 1, wherein the small orifices in said wall portion are of conical cross-section adjacent to the exterior of the wall portions.
 8. A forming chamber for producing a mat of fibers on an air-permeable support at the bottom thereof with suction means below said support, comprising a. wall portions above said support to define said chamber adapted to be maintained at subatmospheric pressure, b. an inlet for the fibers at the top of said chamber where-from they are distributed onto said support in a downwardly moving gas stream, c. said wall portions having a plurality of openings along a substantial height thereof, through which a plurality of small air currents are aspirated into said chamber and thereafter deflected in a downward direction by said downwardly moving gas stream to form air cushions adjacent to said wall portions on the interior of said chamber, along which the fibers slide downwardly without contacting and consequently adhering to the walls of said chamber, d. said openings in said wall portion being of circular cross-section of diameters ranging between 2 mm. and 6 mm. with an underpressure in the chamber amounting to approximately 25 mm. to 35 mm. of water column, and correlated in their distribution and spacing so that uniform air cushions are formed on the interior of said chamber without the generation of vortices therein.
 9. An apparatus as set forth in claim 8, wherein the diameters of said openings are approximately 4 mm.
 10. An apparatus as set forth in claim 8, wherein the spacing between the openings in a horizontal direction is approximately the same as in a vertical direction, said spacing being at least approximately 7 mm. between the edges of the adjacent openings and substantially no greater than the lengths of fibers dropping in said chamber.
 11. An apparatus as set forth in claim 10, wherein the spacing between the edges of adjacent openings is substantially no greater than 52 mm.
 12. An apparatus as set forth in claim 10, wherein said openings are disposed in staggered relation in successive rows.
 13. An apparatus as set forth in claim 10, wherein the openings in the wall portions are arranged in horizontal rows, with the diameters of the holes decreasing from the upper rows towards the lower rows.
 14. An apparatus as set forth in claim 13, wherein the diameters of the openings decrease in each successive row.
 15. An apparatus as set forth in claim 13, wherein the diameters of the openings in groups of rows decrease in each successive group. 